Production of chromium compounds



Feb. 25, 1947. M. J. uDY 2,416,551

' PRODUCTION OF CHROMIUM COMPOUNDS Filed June 22, 1942 CHROM! TE ORE l V l 5IN TH? M77/ UME '/200 "C 'za /350 "C EACH/NG EACH W/TH WATER MUT/0N 0F 501 /05 SOD/UM INVENTOR COMPl//V' y l/ff AMW/N J my Pm, y ATTORNEYS relatively Patented Feb. 25,1947

UNITED STATES PATENT OFFICE .l y l l A2,416,551

2,416,551' l .t ,PRoDUo'rIoN'oFcHRomUM COMPOUNDS n l t Marvin J. UdyQNiagaraFalls, N. Y.

Application June 22, 1942, serial No. 447,963

14 claims. (01..-23-56) i f This invention relates to chromium recovery and has -for an object theip'rovision of animproved method or process'for recovering Achromium from chromium-bearingv materials such as chromite ore. contemplates `the provision of(` an improved method or processifor producing` oxides of chromium as such or in chemical combination with basic oxides such as sodium-oxide Land calcium oxide. Another object of the invention is toprovide certain improvements in methods or processes for producing chromates. A further object of the invention is to provide Vcertainimprovements in methods or processes for producing chromic oxide as such'or in chemical combination with a basic oxide 'such as calcium oxide.v

The invention also contemplates the provision of an, improved method or process forrutilizing alkali metal compounds in obtaining-relatively pure chromium compounds. from crude chromiumbearing materials suchas 'chromite ore.

"f,Throughout the specification'A land in "the claims, the term"chromite ore is intended to include naturalchrom'ite ore, chromiteore conf4 centratesland similar chromite' mineral-bearing materials. y i According to some heretoforer customary proc.- esses, chromite ores'are subjected to oxidizing or roastingtreatments in thepresencev of one or more lalkali metal compounds'such for example, l

assodium carbonate, sodium hydroxide andsofdium sulphate under suchV conditions as to o xdize the trivalent chromium of the chromite ore to the hexavalent state and produce alkali metal chromates such, for example, as sodiumchromate. Sodium chromate thus IJlfQduced is separated from the product of the'roastng or oxidizing treatment by leaching andcrystallization and marketed as such or converted to sodium -dichromate for marketing. ..Chromium'rcompounds such as chromic oxide," calcium `chromite and *calcium chromate are producedlby-suitable reduction or conversion. treatments of commercialz-vsodium chromate' orsodium dichromate. "Usua11y,-tne f production of such compounds isca'rried out independentlyof the chromate production opera'- tions and, consequently, the sodium oxide content of the chromate compound 'is wasted or, at any rate, is not recovered in a form in which it is readily available ffor fre-use in the chromate production operations. Therefore;A chromicoxide,'calcium chromite and ycalcium chromate are costly* commercial products.' Such More particularly, the invention y. f

many industrial uses, but their `usefulness is not fully developed because of high costs. l The-present invention provides for `the production ofsuch compounds under conditions and according to procedures involvingeicient Aproduction Aoi alkali metal chromate and utilization ofthe alkali metal oxide content of chromate einployed'in the production of the'compounds for producing additional'chromate. A completepreferred process of the invention comprises the following steps:

(1) Preliminary heating of a charge of chromite ore and calcium oxider in nely divided admixture to a temperature sufficiently high, to sinter or melt the constituents thereof. i

v4(2) Digestion of the product Vof the sintering treatment in iinely divided form with an aqueous solution of causticlsoda ors-sodium carbonate or both, and separation of the resulting aqueous liq- ,uidl from residual solids. v

' (3) Oxidation or roastingvof the residual solids from the digestiontreatment in finely divided form incontact with an oxygenfcontaining gas l.

such as-air and `in the presence of one or more alkali metal compounds in. accordance with well known oxidizing or roastingprocedures to produce alkalimetal chromate.

(4) Separation of the-chromate from other materials associated therewith in the oxidized :or roasted charge as, for example, by leaching with an aqueous liquid followed by evaporation'and crystallization.

(5) A concentration treatment, if desirable or necessary, to recover unoxidized chromitewhich may. be returnedto the process.

' (6) 4Reduction of the chromate directly or after Conversion to dichromate with the production rof chromicoxide as such, or chromic oxide chemically combined withfa basic oxide, and one or more alkali metal compounds suitable vfor use in the oxidation or. roastingltreatment of the rey sidual solids from'the digestion treatment. s (7) Utilization of thev one 0r more alkalimetal compounds thus produced in an :oxidizing or roasting treatment of the residualsolids from'the digestion treatment or partly for that purpose and partly for leaching" chromate-bearing material.

A preferred complete process'of the invention also may involve thetpuse of the aqueous liquid from the digestion treatment for digestion of additional material, for leaching the chromatebearing product of the oxidizing or roasting treatment or for both purposes and useof mother liquor vfrom. the evaporationjandf crystallization treatment.. for leachingthe chromate-bearing product of the oxidizing or roasting treatment,

, for the digestion of fused or sintered material or for both purposes.

Thel invention will be better understood from a consideration of the accompanying flow sheet,

which illustrates one preferred method or process of the invention, in conjunction with the follow- Y ing discussion and description. l

The present invention is basediin part. on my discovery that chromium-bearing materials of the nature of chromite ore can be made more Y accomplished.

strength of solution employed Will Vloe determined to some extent, at least, by results sought to be Thus, for example, if alumina recovery is desired, relatively strong solutions are employed. rWhen recovery'of aluininais not desirable, relatively dilute solutions are employed.

Solutions containing not more than about Vtwenty amenable to treatment for the production of.. chromate compounds if a charge of the ore and lime in controlled amounts' orVv proportions` is heated to a temperature at least sufficiently high to sinter constituents of the charge 'and the re` f sultant production thereafter subjectedj` to a digestion and decomposition treatmentyvith an Valkali metal compoundlsuch, for example, as

caustic jsoda (NaOI-D- or sodium carbonate (NazCOa) .5. rIfhe 'heatin'gzofI the charge of. ,chrofr mite ore and lime/may be-merely sufcientto bring about vasinte'ring of the constituents therepercent by weight of Ythe alkali metal compound can be employed Vto -decompose the fused products V effectively vvithout dissolving any vsubstantial amounts of alumina' contained therein. Solutions containing labout .twenty-five percent or more of the alkali metal compound can be employed to dissolve alumina effectively with the production of, which maybe accomplished ata temperature of about'. 1200*? C., or between 1200 C. and 1350:

C.; or may -be sufficiently intense to bring about ainelting'of theol-large. The fusion treatmentY results in the formation of i analtered orsubstituted chromite-by bringing magnesi'a or lime or limeand magnesia into chemical combination With chromic oxideV normally chemicallyrcom-` bined With Yferrous oxide, and inthe production of icalcium'compounds of iron, aluminum, chromium Vand silicon such, for example, as calcium ferrite, ycalciumV aluminate and calcium silicate. The fusion treatment also is capable of concen` trating chromium-bearing fminerals into relatively largerV particles of high specific gravity` which may-be separated from associated gangue` '40 The digestion andAY decomposition treatment i materials byV ordinary concentration methods.

of sodium aluminate in solution. In treating ores h igh in alumina, I'preferto employ strong decomposition solutions to produce solutions of sodium Yaluminate from which aluminamay be recovered by standard methods. In` treating ores loWin alumina, or Wh'enalumina recovery is not cornmercially feasible, I prefer to employrelatively dilute decompositionsolutions-.containing about ten 5to vtwentypercent by Weighty ofthe alkali metal compound, Caustic sodaand sodiumcarbonate may be employed With about equal effectiveness. i l

Afterdigestion of the fused product vvith the decomposition solution, lthe product is ltered or decanted and filtered to separate the solution from thesolid decomposed. material.` The solid l residue Imay be dried andmixed With lime Vor with an alkali metal compound appears to effect 'physical decomposition ofv calicum compounds of f iron, aluminunnchromium and silicon Vformed in, the fusion `treatment' with theproduction of sim` ple oxides of those. elements.

Products resulting from the Yfusion and de-r 'composition treatments vof the invention may be 'xidiz'ed'eiectively in` oxidation Ycharges contain.`

ingsubstantially smaller amounts of lime or other diluent material than .the amounts requiredto lime" and'soda ash, by grinding if necessary, to prepare a chargesuitable for' oxidation. f YSeparation'of the digestion solution from thesolid 'mate- Vrials may be socontro'lled'asV to provide for reten- Y tion Yin the solid residue of suilcient sodium com.Y

pound to form sodium chromate with all or a large proportion of unoxidized'chromium remainl ing in the solid residue. The separated solutionV may befv returned tojjthe process forjfurther 'use either directly or after suitable" treatment for the recovery of any dissolved values such as valumina or sodium chromate which maybe formed as the resultof maintaining oxidizing" conditions during .digestion "or during Yfusion anddigestlon, or the separated solution may be'employed for leaching prevent fusion in charges comprisingotherchromium-b'earing materials Ivcommonly employed may be carried out' with oxidation chargescom- 'prising products resulting from .theff'usion and decomposition treatments of 'the invention and.

containing A lime or otherdiluent material in amount not substantially greater than about fifty ofA charges comprisingl natural chromite ore.V

Theproduct of the fusiontreatment `may be i subjected-1 to ltheV 'digestionl and decomposition treatment with an alkali metalcompound in any vsuitable manner.- In a preferred process ofthe invention, the product of the fusion treatmentis f for chromate production; VEffective, .oxidation digested with acaustic aqueoussolutionof the alkali metal compound at--a boiling temperature. The product of the fusion treatment'preferably i is vground to form-afmely fdividedfproduct consistingl'argely of particles small enough .to pass 'a 10D-'mesh screen prior to being subjected to -the decomposition treatment. #Digestion 'of the. prode 1 uctfef the fusioni treatment :may be. carried `out the chrom-atebearing productV lresultingY from residue from digestion. y

Fusion of a charge containing `lime andY chromite ore'may be carriedou't atarelatively low temperature of incipent fusion vat which va product in the form of-clinker Willbe produced, or,`fusion of the charges --may b'e carried out; at a relatively hightemperature atwhi'cha product inthe form subsequent 'oxidation or roasting ofl the solid of a molten bath Willbeproduced.y sinter-ing or incipent fusiongmay'beicarried out at tempera- 'turesin' ther range1f1200.(3..A to .1350". C. Fusion treatments .of the inventionimay: be carried out in .anyssuitable typegofgfurnace `or heating equipment.;` For heating..charges. to. temperatures oi Y incipient fusion, I prefer to employ combustion y" heated furnaces of the-type of cement kilns, and,

for heating charges Vto melting temperatures, I

prefer to remploy .the submerged arc type electric furnace. Other types y o f combustion Vfurnaces which may beemployed'for fusion 44Ainclude reverberatories, cupolas andblastvfurnaces. n prac- .coing thenvention, .fils nef gehalte@ @empriS- me..g1.aY.-,-b .9W-ie@ 91,111

ing cnromite. Oregnnd under neutral, oxidizing or reducing conditions.v

If fusion of the charge is carried out under con.-

known in the metallurgical art. When the fusion` treatment is carried out at a relatively. W temperature of incipient fusion and under reducing, conditions, the metallic iron formed will be distributed in the form of small particles throughout the residual non-metallic material containing the unreduced chromium. Separation of the metal particles from the non-metallic material may be accomplished by any suitable means.

The amount of lime employed in the fusion treatment may be controlled to form calcium compounds of the iron oxide, alumina and silica associated with the chromic oxide of the ore and to form calcium chromite with any portion or all of the chromic oxide of the ore. The alkali decomposition treatment of the invention effec-v tively decomposes calcium compounds, such as calcium ferrite, calcium aluminate, calcium silicate and calcium chromite, formed in the fusion treatment with the production of simple oxides such as ferric oxide, alumina, silica and chromic oxide.

v In preparing chromium-bearing materials '-fo'r subsequent treatment by oxidation methods to 6 @moneta-'itis decomposed subsequently byI heat with the Production of calcium'oxide." g r`Theadvantages of the fusionA and digestionj featuresof the `process ofthe inventionA are il lustratedby vthe following exammosr .A quantity of sintered product was prepared by formin'gvanv. intimate mixture consisting vof finelydividedchromite ore of the following comproduce chromate compounds,` I may employl lime in any'suitable amount. The use of lime in an amount in excess of that equivalent to two molecules of calcium oxide for each molecule of chromic oxide, iron oxide, aluminum oxide` and silica permits the solid product of the fusion and decomposition treatments to be subjected directly to the oxidation treatment without the use of additional lime and with substantially complete conversion of the chromium `to the chromate form. All of the lime'required for a subsequent` oxidation treatment may be added in the charge subjected to the fusion treatment or a portion maybe added in the charge subjected to the fusion treatment and the remainder maybemixed with the solid product of the decomposition treatment prior to commencementl 'of the oxida-l tion treatment; j In view of widely diierent characteristics of chromite oresfrom various sources, itfusuallyisv advisable to conduct preliminary laboratoryitests in order to determine the charge compositions and operating conditions which will produce optimum results in plant operations. .l In processes involving melting of the charges, the original charge materials may be employed in any suitable particle sizes. In processesy in volving sintering (incipient fusion) of rthecharges, the charge materials preferably are employed in the form of particles small enough to. pass a 100-mesh screen or even small enough to pass a ZOO-mesh screen. (Screen sizes referred to in this application are based-on the Tyler series.) Lime may be incorporated in the fusion and oxidation charges initially as calcium oxide or in the form of ylimestone (calcium carbonate). when the lime is employed. initially .calcium position and CaO) in the proportions 100 parts of ore to 100 parts of lime stone, by weight, both the ore and rthe lime stone being ground to minus 100-mesh,

and heating the `mixture at a temperature of vabout 1300 C. for a period of time sufficiently long to accomplishsintering, and the sintered product,-l after cooling, was ground to form a. finely divided product consisting of Vparticles small enough to pass a 100-mesh screen.

Ore analysis f Per centi CrzOs-V 44.00 FeO 23.60 A1203 13.80 SiOz '6.20' MgO 8.30

CaOV

d Example I f A portion of the Yfinely divided sintered prod-l uctwas digested With an aqueous solution of sodium carbonate (NazCOs) containing twenty percent (20%) sodium carbonate by weight at a boiling temperature for a period of about one hour.'v Digestion was carried out in an open vessel with the surface., of the pulp in contact with atmospheric oxygen. The digestion pulp was filtered, and the ,solid residue was dried and intimately mixed with limestone and soda ash in amounts and proportions equal to 30 parts by weight of lime stone and,1'7.5 parts by weight of soda ash' foreach 100 parts by weight of chromite ore incorporated inthe portion of the sintered product 'l C. for about one and one-half hours.

`The amountof soda Aasl'i employed in roasting` the residue from the digestion treatment was about that required to formv sodium chromate with one-half of the chromium present in the residue.

i In the digestion treatment, about 15 percentof the chromium `of the originalchromite ore was converted tov soluble sodium chromate. In the roasting treatment,v about percent of the chro-f mium contained inthe` residue was converted to chromate. v v

' Example' II A second portion ofthe finely divided sintered product was digested with an aqueous solution of sodiumV carbonate (NazCOa) containing twenty percent (20%) sodiumcarbonate by weightl at a boiling temperature for a period of about one hour. Digestion was carried out in an open vessel withthe surface of the pulp in contact with atmosphericv oxygen. Thedigestion pulp lWas lilteredand the solid residue wasvdried and intimately mixed with limestone and soda ash in amounts and `plopiirtons equal to 30 parts by Weight of limestonev and 35.parts by weight of soda ash for -'each;100parts by weight of chromite ore incorporated in the portion ofthe sintered productsllblected to digestion. The mixture thus produced: was roasted" in air at'aboutl 850 C. to 1909 .C., 'for :about: one .and "onefhalf' hours. f' i finely divided lime stone (53%-r mium containedin the charge.

- ploye'din Toasting the residugrrom'jltne -=digesuon1treatment --was apoutithat required tolifor-m sodium -chromate witnlau 'ort-he'ehromiumf'present 'in' theres'idue,

In the digest-ion 'ftraftnient,- ab'out 15 :percent olthe `lnomium of the origmal chromite fore the roasting treatment, about A 9855'percent-of the' chromium Vcontained nthe residue Kwas converted i @remata f mamma-IH- Ath'ird portion of the finlydivided'sfinterel ntima'te'ly'vvit'h4 limestone and productfwas mixed -i sjodaas'h in amounts and proportions equal'to 3U parts by Weight of limestone fand"r ,1755 parts `-by Weight ofsoda lashfor 'ea-ch 100"pa1jts v-by Weight of chromite ore incorporated in Ithe portion `of the sintered productzemployed. The mixture thus produced was roasted in 'air at about 850 C, to 1000 C. forabout .one and .onehalf hours.

' I n fthe roasting treatment approximately .sixtytwov percent (62%) of the chromium containedin the :Charset/as YconvertedY ,to chromata "As in Example I theamount ofsoda ash emev ployed in thecharge subjectedtothe roasting treatment Was about that required to form sodium chromate with about one-half of the chro- ,Example-IV l thesintered productemployedl The mixturethus produced 'was .roasted in air at about f850 C. to 1000 AC. for about one fand lone-half fhou'rs. In the roasting treatment approximately eightw'two upercenti 82%1) Vofthe uchromium contained Vinthe-charge vWas converted to chromate.

Asf in Examples VIand VIII, the amount fof -soda ash employed iin the charge subjected to 'the roasting treatment `was 'about :that required to form v:sodium ichromate with iaboutonerhalf-of the chromium contained :in 'the charge.

' yEacanpl'VV A 'fifth portion of theiinedy product wa'sfmixed intimately .with limestone and soda; ash in amounts and proportions equal to150 parts Joy .Weight-of lli-mestone and B -partsV -by weight .ofksodaxa'sh .fforlea'ch 1'00 Vpartsfby"Weight of chromite ore incorporated in the portion ofthe sintered product employed. The mixture thus producedj was roasted lin air at about 850 C. to

Q0 C. -for about one :andfoneha'lf hours,

"-I'n" the roasting treatment approximately 3 ninety-seven'and one-half '(97.5-9l7?) vpercent of the chromium contained'in vthehar'ge was vcon-l i verted to chromate. `v s v s f vAs in "Example II,v the amount of soda ash employ'ed inv thecharge subjected *tothe roasting treatment was vabout th'atrequired "to 'form soi tained in the charge. Y 'The' advantages vof the' digestion and decomposition treatment .fof -the invention in conditionring the 'product .of the "fusion treatment fior -subdivided fsintered plying composition following fusion andfwltli' conditions otherwise lthesame'as'inExample V111,5 total conversion to chromate in excess ofninet-yfive percent v`was accomplished as cmparedjWith-sixty-two percent (62%) conversioniin carrying out the'procedure ofExample III. Mod'-v icat'ionof the procedure Aof Example'III by the inclusion'ofa relatively large amount of ylimefor diluentf-purposes resulted in `raising conversion s theinclusion of sumcient soda ash to form sodium chromate 'lwith'all of the chromium present'in the roasting charge instead oi `with only half ofthe chromium .as inlExample I; Modification of the procedure of'ExampleIII'bythe inclusion of sufricient .soda ash .to :form sodium'chromate Withlall of the ,chromium-.present in the charge required the inclusion :also of arelatively large amount of diluent lime, as indicated by Example V Inthe procedure of AExample V, roasting with La. lower proportion of lime was not :feasiblerbecauseof sintering induced bythe presence ofthe large amount of soda ash. Y

Digestion of the sintered product under oxidizing conditions in accordance with the invention performs the three-fold function of .(1) providing .for :rapid production and .recovery of a.sub stantial proportion of thechromium .of lchromite ore in the'form of soluble alkali `.metal chromate.; (2) decomposing chromite ore and making'the chromic oxide content more readily amenable to oxidation Vby roasting in air, and l(3) effecting re-V agent 4and operation economies by eliminating the necessity of employing large Quantities of w low temperature in the neighborhood of about 120.0 C. Y A further advantage of the digestion and decomposition treatment of the invention resides in -thevfact that effective conversionto chromate Y Yuse-of relatively maybe accomplished With the small .amounts of .soda a'shg/lime in the .charge providing .the base for combination withsthe chromium .trioxide produced. .Elective conver sion. to chromate may be .accomplished with charges comprising the decomposed. product, ,lime andjsoda ash in which the amount ofsoda as'h does 'not exceedV 'that required for forming sodium chromate With` about tengpercent of the Ychro` mium offthe charge. Charges containing Soda ash in amount equal to that requiredforfrm-f ingsodium chromate `With about Vfty. percent of the vcharge may be oxidized readily `andsubstan-v tially completely. The use of relatively@small amounts of soda ash in oxidation charges, is highly vadvantageous because of elimination of fusingfandballing difficulties encountered in oxi dation of Acharges containing large amountso'f soda ash. s Y

-In treating thesolid productsgof the v'fusion andv vdecomposition treatmentsdirectly to -produce chromates, I `prefer to form finely divided admixtures of the products with lime or lime and 11090" C. until the desired 'degree or'amount of conversion' 'to chromate has be'en effectecLI may tions," depending. .upon the end.v product sought to be obtained. Iffsodium chromate is-vsoughtf, I may employ soda ash suflcient .to formfsodium chromate with all of .the chromium vpresent.. If an alkali metal bicliromate vis sought, 4I may employ soda ash .sufficient .towform sodium rchromate with about half ofthe chromium present and lime in excess.. of that requiredhtoform calcium vcliromate IWith the remainderfof the .A

In a complete jprocess ofuthe invention', the v. .1

roasting treatment isv leached with the aqueous liquid from the digestion treatment containing caustic soda'in substantial and predetermined amount. The complete' inyentionisl based in part on my discovery thatl sodium hydroxide functions as a salting-out agent for.. sodium chromate when present in an aqueous solution of sodiumv chro. mate duringl .evaporation treatrr'ient. of thev solui tion. I have discovered;also, that the .presence natu . Leaching and evaporation e'iciencies ofthe process of the inventionare illustrated by the following table of results obtained in solubility testsL carried out at room temperature (about f1 0 F.) employing 'pure' water and Water solutions o f` sodiu1 n, hydroxide as solvents forl sodium chromate:

. i. v i Gramsr of sodium Percent NaOH in chromate (NazCrOo orignalsol'ution per liter of final solution r Final solutions 'vwere preparedby saturating'the original solutions `Withfsodium .chromate at teinperatures substantiallyv higher. than room tem-` peature andthereafter cooling the solutions to of sodium hydroxide in y leaching rliquors`".in t.

limited amounts up toabouttwentyiiw to tl'iirtl percent by weight does, n otinterfere `with ,the leaching operation; th'atleaching `liquors con;` taining sodium hydroxide in such amounts can be employed substantially as i -effectively .as pure water to produce nal`.leaching' liquors or solutions of sodium clirom'a'teI containing sodium chromatein amounts equivalent to or greater than the amounts containedlin final leaching room temperature to permit crystallization of excess sodium chromate to insure the -production of saturatedfnal solutions. l Y .A

In Nthe preferred 'process .of/the' iny'entiori`..1 treat. the crude sodium chromatefbearingmatee ,rial obtained by roasting the solid .residue from the vdigestion treatment 4with an aqueous solution of-sodiun hydroxide :comprising the vaqueous solution employed for' digestion of the vproduct of the ufusion `treatment and containingsodiurn hy solutions .or liquors Iproduced in commercial processes of thejtype employedv heretofore. I have found thatleachin'g-.liquor in the form of an aqueous solutioncontaining'fan amount of sodiuml hydroxide tupfto -about vthirty percent of the Weight of thewaterin thesolutionLcan be employed efficiently to .produce a'flnalleach. ing liquor or .solution containing sodium chromate in an amount about inthe range. 150'to 400. grams .per Aliter. of solution.3 and .that an evaporation Vtreatment of. the solution sufficient to raise theconcentration of sodium hydroxide to about thirty-five tofcrty percen'to'f.` the Weight of the waterin the'v solution .will result inv precipitation orcrystallization from solution of froml about thirty-meto' ninety' percent of the sodium' chromate at a temperature of. about .25." C.-

Thus, for example, 'an aqueous solution of sodium hydroxide can .be employed 'ellcien'tly to `produce einmal leaching liquor, containing morethan four hundredgrams off sodium.' chromate per liter of solution, and evaporationjofdan amount of Water sfuflicient to raise the sodiumhydroxide content to forty percent olfjthe Weight o f thewater in the Solution. .will result.' 11.1 precipitatidh or Crystal-i liz'ation of sodiumjchromate 'corresponding to about threej'hundre'd iitygramsperjliter of I,the finalleaching solution upon cooling ofithe soluf tion to about 25 C. v'lfhus,.evaporation oi less than fty percent ofthe water results in precipitation of about ninety percent of the dissolyed `sodium chromate. Evaporation of yftypercent of 'the' water"1 of a neutral'aqueousfsolution of sodium chromatef containing Vthe :same amount of sodium 'chro'rn'at'efwould 'effedtthe precipitation or crystallization of little or no sodium chromate; 11,

droxide inamount k such as toproyide for effective leaching with'the production of a iinal leaching liquor containingthe-desired amount of sodium chromate.`v I then separate the leaching liquor froml insolublefmaterialA contained in the crude chromate-bearing materialand heat the leaching liquor to evaporate suiicient water to increase the.l sodiumI hydroxide concentration of the re.- fm'ainder.. oik the solution to a point at which, upon cooling of the'sol'ution, a desiredproportion of thesod-lum chromate of the original final leaching'solution Will be obtained inv solid form. The resultingv solution! is .separated from ythesolid S07 d'ium chromate and utilizedafter dilution to the desiredextent .With u/ater,4 partly for leaching additional sodium chromate-bearing material and partly for` digestion of .additional fused or sintered material. It is unnecessary to carryout complete evaporation .of the. leaching solution.- .The presencein .theleaclnng solution. of residual'sodium chrom'ate fdoesI not [interfere with leaching. Evaporation needbe carried out only to theextentthatthe amount of residual sodium chromate isV not.suiiciently4 large to form a burdensome `load inthesystem; f v t .j 'Leachin'g of the crude chromaticbearing mater rial may be carried. out at anysuitable temperaL ture. Temperatures above about 251 C. may be employed satisfactorilr j Ijprefer to. Conduct-the leachingioperationat a temperature between 25 C.' andthe boilingLtemperature of the leaching liquor, a'temperatureiin the range 90 Clto 100," C.; vbeing satisfactory'4 for most practical purposes. A' InA apreferredcomplete process of the invene tion, `I`leacnthe roastedcharge with an aqueous solution of sodiumflj hydroxide ,containing about twenty to thirty percent by'weight of sodium hydroxide.y Leach'ing preferably isconducted under conditionsfsuch aste produce a nnal leaching liquor containing 'aboutjthree yhundred to vfoin" hundred grams ofsodium chromate'per liter be'- y cause of practical. censiaeratmns. The., :mai

leaching liquor is--heat'e'dl to Veffect evaporation of.

suflicient Water toproduce upon cooling. tofabout' V25?` C., a, solution containing lessi. than one hunamen , '12, ot the chromite ore and limeor other .diluent material vin amount equalize4 *as*v muchas three f times the weight of therchromite orey In the.

liquid isE separated from the; 'soltLsUdium chro mate, diluted v:with an amount of water corresponding to that evaporated and returned to the process for use in treating additional material.

Instead of 'relying solely uponi cooling '0fY the solution after evaporation to effect precipitation vor crystallizationv of the sodium chromatic, I may continue the evaporation treatment Ato the point at which precipitation or crystallization takes place during the. course of evaporation-` In such a case,. Ipmay effect further precipitation or crysf" i tallizati'on by'coolingiafter' evaporationn or' I' may dilute the mother liquor-for refuse immediatelyv after separation' of the` lsolidiscdiumchromat'e. For-'separating the` soli'd'sodium chromate from themother liquor, I prefer to employacentrifuge ing operation; y y

An additional advantagenfftlie-'process-'of the invention over heretoforewcustoinary processes resides inY the fact-that thfec addition of an, agent likeK sodium'carbonate to the" leachingl liquor to convertl diflicultly` solubleca'lcium chromate to soluble Vsodium chromateis' unnecessary. The sodium hydroxide o'the` leachingflquor' employed in the,v process otherinvention` reacts with any calciumY chromate contained inthe crude chromateebearing material tol form sodiumy chromate and relatively insolublev-calciumhydroxide. The

' danger of carbonization'of the caustic soda ofthe leaching liquor bycarbon dioxide of the atmosphe're-is eliminated by-virtue ofthe presencev in the crude chromate--bearing material' ofr caustic.

lime-which functionsto causticize the sodium carbonate-Whichmay be'formed during the course ofrthe'process. l Y The solid residue remainingafter leachingoff the product ofi-the roasting treatment 'maybe wastedroir subjectedtoa concentration treatment tcseparateandrecover-aI concentrate containing unoxid'ized orundecomposedchromite; Concen-l bekept porous. tio' permit' eiTct'ive-.contact of and,

oxidizing gas suclrasair with; the solid` compo` nents. of. the; charge. According to. someV hereto',

forev customary practices; porosity, isi maintained f .by 'dilution oi' the. charge with a, refractory subtemperature.A to. avoid.' fusion. Customarily;

`charges employed; f'o1'\,..'thesAv production ofJ` alkali metal. chromatesli by.` ain oxidation or 'roasting comprise natural chromite ore; alkali metalcomf stance such?, asrlime (CaO.) and; by. control of i treatment of such cl'iar'gesv at the hightempera,- tures requiredforoxidatiomA even with the large amountsv of' diluent material' usually included, it is difficult to avoidfusion with consequentre-v duction. in the eiliciency) of oxidation, and, even under optimum condtijon'so' operation, it i'sldifii'-V cult or impossible to approach oraccomplish com# pleteAe oxidation: `Satisfactory oxidation" usually requires heatingA of the; charge;y under: oxidizing conditions for many hours; Y ,Y

Fusion of the roasting ,cl'rarg'egx i1'1""some` cases; mayf-be attiibutedgtc'someextent'to thepresence in thechargev ofthe largejram-cunt of? relatively;

lowemel'ting'point alkalimetal'compound required to: form alkali metalchromate with allot the chromium of the chromiteV oreand tothe produc-v tionzof" a.; relatively large amount' of low-melting. point alkali' metalchromate inv such cases. sion may loel avoided toak large' extent by' employing the alkali metal compound in an amount substantially smaller than that required to form alkaliv metal chromate withv all of 'the chromium of the chromite ore' and thus'avoiding' the inclusion and production of large amounts oi'relatively low-melting pointcompoimds. Ineiective or in-V complete oxidation may' be'attributed to the exclusion ofV the oxygen vof 'the' air` from contact. with the; chromite particles resulting' from agglomeration. ofthe charge particles or coating ofn` the particles witlr alkali metal chromate pro# duced in the roasting treatment. In employing. rotary' kilns and heartnfnrnaces' provided with rabbling meansY in accordance with heretofore customary-procedures; conversion to chromate of about eighty percent '(80%)' of' the chromium of the chromi'te ore4 ofn a' roasting orA oxidizingy charge can be accomplished quite eil'icientlir, but higher degrees of conversion. are. diilicult' and relatively expensiveto accomplish; d

The concentration 'feature of the present. in' vention provides imprcvementsby' meansv of which. less: dilute oxidation; charges; or charges. more concentrated. with respect to chromium contentLmay'be employed to.' effect improved recoveries' ochromium in the' form of chromaten shorterY periods' of time. and AWith greater ease` of' handling and` greater economy'resulting from'tliev useof' the more concentrated' charges.

d The concentrationfeature of the invention is based. in part:- on myj observation that oxidation of the chromium ochrmite`bearing material proceedsV rapidly during the' early stages oftlie oxidizingv treatment an'd'on my discovery that,

at' the conclusion ofthe' oxidizing treatment', the

example, thatin a given charge,` containing lime, cl'iromite` andvv an alkalhmet'al.' compound" such asv sodium, carbonate; iiiwhicl'i. a. maximum of about .ninety percent (901%.)- "conversion, toA chromate can he. eiected in twelve? to. finurteen hours, a

if conversion. ofi asmuch asf eighty percent.

iseflectednwithin about three hoursor less.

L have found" that'. about the same` amount of chromium Will be. oxidizedv in. theinitial. stages.

cf .thetoxid'ati'on treatmentvwhether the amountV of alkali metal compounch'employed, isl suilcient,

Vtti-.form alkali metal chi-tomateY Withalll ofthe;

chromium or WithK only aboutseventy toleigllty'v i' percent (970' to 80%)1-v of; the'c'hrorrnum.`

.According to the present invention, the tend-Q .13 enoyl of the charge to oxidize Veffectively with a limited amount of alkali metal compound may be taken advantage of` to provide an oxidation charge (containing a limited amount of alkali metal compound) which sinters or fuses. less readily under the conditions employed in the oxidation treatment, andthe fact that the unoxidized chromium remains as a lconstituentof the original chromite may be taken advantage of to recover the unoxidized chromium by means of a concentration treatment. The use of a relatively small amount of alkali metal compound in the oxidation charge also permits the use of smaller amounts of diluent materials suchuas lime, and charges containing limited amounts of diluent material and alkali metal compound may be oxidized at higher temperatures with less danger of undesirable sintering or fusing resulting and with consequent increase in .the rate of oxidation of the chromium. The -components of the charge may also be employed in coarser states of sub-division. Thus, the use of a limited amount of alkali metal compound permits the use of substantially more concentrated and coarser oxidation charges which facilitates subsequent leaching and filtering operations and improves the efliciency of the oxidation treatment by reducing the time of oxidation. Charges consisting largely of particles small enough to pass a 1GO-mesh screen can be treated substantially as efllciently in the process of the invention as charges consisting largely'of much smaller particles can be treated in processes of the type employed heretofore. .The invention permits the use of smaller charges than those employed in heretofore customary processes to vproduce the same amounts of chromate. o ,o

. Oxidation of the chromite ore in the presence of basic materials suchas lime and alkali metal compounds results in decomposition of a large proportion of the chromite mineral of the charge with the production of ldecomposition products which have apparent specific gravitieslower than the specific gravity of chromite mineral remaining undecomposed with the result that the undecomposed chromite mineral may be separated from the solid materials associated therewithby gravity concentration methods such, for'example, as tabling and water classification. Compounds such as iron oxide, silica. alumina and magnesia initially associated with that portion of the chromite mineral which is decomposed in the oxidation treatment are converted in whole orin part into compounds of sufficiently low specifiegrav# ities or of such physical forms that they may be floated or washed away from the particlesof undecomposed chromite mineral 'by vmeans of Water. My investigations have indicated that iron is the most diiiicult of the original constituents of the chromite mineral to separate from the undecomposed chromite mineral. `The chromite mineralv recovered by concentration may be contaminated to some extent with iron in the form` of ferrie oxide.' If desired, this contaminating' iron may be removed by treating the con# centrate with an acid such as sulphuric acid'ir aqueous solution.

The solid insoluble residue remaining after separation of the soluble chromate may be subn `iected to a Water classification or concentration treatment; preferably Vin the form of a table concentra-tion operation in Whichthe vundecomposed chromite is separated by gravity from thelighter decomposition products yassociated therewith'.

'I 'he concentration. `"lf'eatment results in the recov-:f

ery in the form of allow-volume conceritratA` of ninety percent or more `of the'undecomposed chromite. The chromium content of the original or rough `concen'trate is only slightly lower than the chromium content of the original chromitesubjected to oxidation. A clean concentrate containing about' thev same chromium content as that of the original chromite may be produced by treating the rough concentrate with an acid such as sulphuric 'acid to remove iron oxide associated with the chromite particles.

By concentrating the residue remaining after separation of the soluble chromate from the oxidation charge and returning the concentrate to the process for oxidation to chromate an overfall recovery of chromium as chromate in excess of ninety percent (90%) can be accomplished iconsistently. 'f l Y According` to the Iinvention, sodium chromate produced in the process is subjected to a reduction treatment directly or after conversion to di# chromate With the production of chromic oxide, as such, or chromic oxide chemically combined with a basic oxide, and one or more sodium compounds suitable for use in the oxidation or roasting treatment of the solid residue of the digestion treatment.' The one or lmore sodium compounds thus produced are utilized in the digestion ofthe fused or sintered product and in the oxidation or roasting treatment of the solid residue of the digestion treatment. Through the recovery and re-use of the alkali metal oxide of the chromate,

the costs of producing compounds such as chromic oxide, calcium chomite and calcium chromate are reduced materially and a Wider field of usefulnessfor such compounds is pro.-

vided.

According to a preferred form of the invention, sodium chromate (NazCrOi) or sodium dichrof mate (NazCrzOi), in nely divided form, is in.- timately mixed with finely divided solid carbonaceous reducing material and ignited. Finely divided charcoal, coke or coal may be employed satisfactorily as the reducing material. Finely divided charcoal may be employed most advantageously for the kproduction of pure products. Finely divided coke orcoal may be employed when relatively impure products are satisfactory. When coal or coke is employed,the products resulting from reduction of the chromium of the chromate through elimination of a `portion of the oxygen may be contaminated'with coal or coke ash. Myinvestigations ,have indicated that reduction of` alkali metal chromate .and `dichromate with carbon-through ignition-of intimate mix-l tures of the `materialsproceeds somewhat accordi ing to thel following equations:

Namo? +30 Nazoorzoa +300 '(NaizOSrzOa),r sodiuml carbonate and 'carbon' monoxide in reducingrsodium chromaterWhen the solidreaction product resulting from thegre's duction of sodium dichromate' lwith carboni-is digested with water-(heter cold-)9; asolutionsof; sodiumhydroxlde substantially fr ee of-'sdium carbonate and a solidarcsidue..consisting-essen# many-aff,ehmmiehydmxideccrnmizareipm@ residue resulting, from digestion. tor an oxidizing treatment with air at an elevated temperature A(above about 500 C.)E and,l'eaching the resulting product. with Water. The residual sodium oxide is.'V converted to sodium chromate in the oxidizing treatment and the chromic hydroxide is converted tov chromic oxide (YCrzOsl. Y

Calciumy chromitei and sodium carbonate may be produced` by formingand igniting, an intimate mixture of' nely" divided sodium chromate or sodium dichromataiinelydivided calcium oxide and' nely divided,v carbon: It'. appears' thatk the, reactions' proceed somewhat y according tothe following equations;v f

()- Ntcrzoq zcao +-2oV f Jcaoiicrzoa -l- Naoom-Co (e) zNaiorgoya-icao-t 3C t 2(Ca;O)b.C1203 -F ZNazCOs CO1 (7)":2Na2oroifl- 202,0 -F-so -v t 1- i (CaO-)LCrgQs 7+.- 2NazCO3 CO The sodium carbonate andicalc'iu'm chromite produced in accordance with the reactions indicated Iby the above equations may be separated by digestingA the solid reactionv products with Water, not or cold; Calciunroxide mayibe employed as such or in, cheiriical Vcombination with: carbon dioxide as'y calcium carbonate.v H

Equations 5' to; 7A indicate the production of'a compound approximating in chemical' composition di-calcium chromite: Compoundsapproximating` in chemical composition. mono-calcium chromite or tri-calcium chromitey may bef* produced by adjusting the proportions of' chromate or'dichromate and calcium oxide employed.

'All of the reactions indicated` by Equations 1 lov '16 vlrL-'a cornpleteepreferredzpmcessrot theinyentiorr. sodium carbonate,- sodium hydroxide or mixitures of ssodium,carbonatel and sodiumk hydroxide obtained by digesting the` solidproducts resulting from reduction. of.V sodium chromate and. sodium dichromate are'employed. in one on more" of' the oxidation or roasting, digestion: and Aleaching treatments; The solutions obtained by digestion of the products .of .reduction maybe evaporated partially or entirely `to:Y obtainv solid. sodium. hydroxide or` sodium carbonateorbotni or toobtain sodium hydroxide, oriv sodium carbonate or' v both partly inthe solid state. andipartly inV solution for ref-use. in thexprocess.V Solutions" containing sodiumhydroxide may. bie passed'in: contact gasesysuch as fluegases, containing carbon diito 6 above are exothermici and usually proceed to completionWithout` the application of'heat from extraneous sources. When the calcium oxide is employed: in thei form of calciumicarbonate` and when the production of tri-calcium chromite is sought,v it! may be advisable to provide-additional. heat: from: extraneous sources to insure eiective chemicalzcombination of the calcium oxide vwith the chromicoxide.

Calcium. chromate may be produced by roasting in air a solid residue obtained in the digestion of a solid reaction product resulting from the reduction. of.' a' chromate ori dichr'omate in the presence; of calcium oxideto. produce calcium chromite.: Substantially complete conversion to calcium chromate may be accomplished by roast'- ing the div-calcium chromite and the tri-calcium chromite. Roasting'of the mono-calcium chromite will result! in the productionv of` calcium chromatej and'chromic oxide; Complete-conver-Y sion to chromate of the mono-calcium chromite' may be' accomplished'by lincorporating in the roasting: on oxidizing' charge at least one molecule ofV calcium oxide. fori each,v moleculetofi calcium. chromtacontained.therein.4 .l f f. Y

sodium carbonate.

chromic oxide.

yoxide to. carbonate the. solution, or' partlyv for the purposeY ofJcarbonating the'. solution and' partly forthe purpose: of; washing 'the gases,A and produce; and convertV the sodium hydroxide to,v sodium carbonate ror sodium bicarbonate or both.

Theproduction of chromic oxide by reduction o-f` sodiumV chromatel with carbon is illustrated by the following example:

A reaction'mixture was formed by grinding together 64'8pounds of sodium chromate (NazCrOi) and 46 poundsof carbon in' theform of charcoal; The mixture was placed in a Crucible', covered With nelydivided' charcoal andig'nited by means cfa gas' flame'. Uponignition; thereaction proceeded quietly andfsmoothly.

The amounty of carbon employed represents an excess of about thirty percent (30%)Y overv that required to eii'ect reduction vof the-chromate; It is desirable that sufcientf'excess carbon be employed to preventV reoxidation ofthe'freact'ion a product during cooling.v A

The solid reaction product vvasA cooled under non-oxidizing conditions, digestedl by grinding in water to form a pulpr to eiect solution of 'sodium compounds andltered. Hot or cold Water may be employed for' digestion, The filtrate obtained was an aqueous solution' of sodium hydroxide and The solid residue, consisting essentially of chromichydroxide (`Cr(OH)3)'Was roasted in air at an elevated temperaturey (1000? C.) to eliminate water and` carbon and convert the chromium hydroxideto chromic oxide.

The chromic oxide-bearing product was leached withy Water to remove soluble compounds such as lsodium chromate contained therein. The residue remaining .after leaching was substantially pure The sodium chromate thus obtained. may be returned to the-process. Convex' sion to chromicoxidefof 97.5 percentof the chromium of the sodium cleromate was eiected.

It will be apparent that' the various novel procedures or features of a completefprocess v of the inventionmay be employed advantageously singly or. inv various combinations-of two or more; The

eiciency sought to beV achieved vin the individual:

steps;i ofi a complete- .proce'ss," Obviously', other procedures than those particularly illustrated.

and' described herein; may Ybe employed for utilizel ing; the:Y various: solutions of" alkali metal: comy pounds advantageously in accordance-with the f invention; Alkalim'etaLcompound required' for The roasted product was 95 p ercentchrornic. oxide.;

making up mechanical losses `may be added at any appropriate stage of the process. Any suitable apparatus may be employed in carrying out the various procedures of a complete process of the invention. Apparatus such, for ex'- ample, as kilns, furnaces, thickeners, filters, dryers, disintegrators, evaporators, grinding mills and reaction vessels may be of conventional designs.

Various individual steps and procedures of the `complete process of my invention are described and claimed in my copending applications Serial Nos. 351,128, filed August 3, 1940, now Patent No. 2,381,236; 401,297, tiled July 5, 1941, now Patent No. 2,381,565; 252,743 filed January 26, 1939, now Patent No. 2,359,697; 401,298 led July 5, 1941; 426,045, led January 8, 1942; and 428,207 filed January 26, 1942. In those applications, I claim broadly the formation of the altered or substituted chromite and its subsequent digestion in a hot aqueous alkaline solution to produce a solid residue in which most of the chromium is still in the form of chromite, which residue, if desired, may be oxidized to chromate.

I claim:

1. The method of recovering chromium from chromite ore which comprises forming a chargev of the ore and a basic compound of the group consisting of lime and magnesia capable of substituting forA the ferrous oxide of the ferrous' chromite of the ore to displace the ferrous oxide from chemical combination with the chromic oxide of the ferrous chromite and form an a1- tered chromite, heating the charge to a temperature above 1200 C., and at least sufficiently high to cause sintering of constituents of thecharge and to produce a product containing an altered chromite formed by substitutionV` of the basic compound for'ferrous oxide of the chromite of the original chromite ore, decomposing the altered chromite by digestion with an aqueous solution of an alkali-metal base, separating the aqueous solution from the residual solid matter, subjecting the residual solid matter, in the presence of an alkali-metal compound, to intimate contact with an oxidizing gas at a temperature sufficiently high to convert chromium contained therein to an alkali-metal compound o-f chromium containing chromium in the hexavalent state, and reducing the alkali-metal compound of chromium to produce chromic oxide and an alkali-metal compound.

2. The method of recovering chromium from chromite ore which comprises forming a charge of the ore and a basic compound of the group consisting of lime and magnesia capable of substituting for the ferrous oxide of the ferrous i pound for ferrous oxide of the chromite ofthe original chromite ore, decomposing the altered chromite by digestion with anaqueous solution of caustic soda, separating the aqueous solution from the residual solid matter, subjecting the residual solid matter, in the presence of a sodium compound, to intimate contact with an oxidizing gas at a temperature suciently high to `convert chromium contained therein to la sodiumcompound of chromium containing chromium in the 18v hexavalent state, reducing the sodium compound of chromium to produce chromic oxide and caustic soda, and returning the caustic soda thus produced for treatment of additional altered chromite.

3. The method of recovering chromium from chromite ore which comprises forming a charge of the ore and lime, heating the charge to a ternperature above 1200 C., and at least sufficiently high to cause sintering of constituents of the charge and to produce a product containing an altered chromite formed by substitution of the lime for ferrous oxide of the chromite of the original chromite ore, decomposing the altered chromite by digestion with an aqueous solution of an alkali-metal base, separating the aqueous solution from the residual solid matter, subjecting the residual solid matter, in the presence of an allzali-metal compound, to intimate contact With an oxidizing gas at a temperature sufficiently high to convert chromium contained therein to an alkali-metal compound of chromium containing chromium in the hexavalent state, separating the alkali-metal compoundof chromium from other material associated therewith in the product of the oxidizing treatment, and reducing the separated alkali-metal compound of chromium to procause sintering of constituents of the charge and to produce a product containing an altered chromite forrned by substitution of the basicV compound for ferrous oxide of the chromite of the original chromite ore, decomposing the altered chromite by digestion with an aqueous solution of a compound from the class consisting of sodium hydroxide and sodium carbonate, separating the aqueous solution from the residual solid matter, subjecting the residual solid matter, Vin the presence of a sodium compound, to intimate ContactV with an oxidizing gas at a temperature sufciently high. to convert chromium contained therein to sodium chromate, separating sodium chromate from other material associated therewith in the product of theoxidizing treatment, heating the separated sodium chromate in the presence of a carbonaceous reducing agent 'to form a product comprising a sodium compound and a water-insoluble compound of chromium, separating thesodium compound from the insoluble chromium compound, and returning Ythe separated sodium compound for treatment of additional altered chromite.

5. The method of recovering chromium from chromite ore which comprises forming a charge ofthe ore and a basic compound of theV group consisting of lime and magnesia capable of substituting for the ferrous oxide of the ferrous chromite of the ore to displace the ferrous oxide from chemical combination with the chromic oxide of the ferrous chromite and form an altered chromite, heating the charge to a temperature above 1200 C., andvat least sufficiently high to cause sintering of constituents of thev charge' and to produce a product containing an altered chro- "chromite by digestion with-an vaqueous `solution l of a compound from the class consisting of Yso- [diumihydroxide 'andsodium carbonate, separat- `ririgthe4 aqueous'solution from the `residual Solid matter, subjecting the Vresidual solid matter, `in the `Vvpresenceo'f'a sodium compound, to intimate "contact 'with an oxidizing 'gas ata 'temperature 'sufficiently high 'to convert chromium contained I`'thereinto 'sodium chromate, separating sodium 'chromate'from 4other material associated there- Within'the product 'of the oxidizingtreatment,

lheating "the 'separated sodium chromate in the 1;

vpre'c'sence of "a carbonaceous yreducing agent to Iforrfnfa product-comprising a sodium compound V"and chromic oxide, separating the sodium com- 'Vpoun'df'from the'chromic oxide-and returning the fseparatedsodiumcompound for treatmentof ad- '-ditional` altered chromite. f

jfhe'method :of recovering hkchromium from i'hromitefore Whichcompri'ses forminga charge If theiore -an'df-a basic compound ofthe group consisting of lime iandrniagnes'a lcapable'of sub-- sti-tilting vfor the ferrous "oxide of lthe Y-ferrous chrdmiteior the lere-luruisplace :the ferrous Aoxide from chemical "combination 'with Ythe lchromic oxide of the ferrous chromite and formian a1- fteredrchromite, hea-ting' the charge Lto La tem'- fperature above 'IZGU/C., and-l at leastsvuilciently high to -cause 'sinte'ring "o'f 'constituents of the "charge V`aridito produceaproduct` containing an aitjer'eu eifromite `formeel 'bysubstitueouv of Ame Dbfasic"compjound' forffe'rrous oxide of the'chromite fof 'the original A'chromite "or,^""decomposing the altered chromite dir grestion l'with an f aqueous Js'olutit'm "off: aicompoundrfrom the-c1ass rlconsisting ofsoidium hydroxide and sodium carbonate, Vs'ep- Iarating fthe' aqueous solution "from the residuali?.

-Vsolid matter', subjectingthe "residual soli'dmatter, -irfthepresence ofaso'dium co'mpound,"to intimate J' z'vorr1ta'uct" withv "an lvoxidizing "gas vat na itemperature siiciently to 'convert v(':lfiromium Vcontained 'therein 't'oisodium `'civ'lromata rseparating sodium "hromat'e from other material associated theref tlith v`in lthe 'product f "the oxidizing V`treatrrren't, ihr-:ating v`"tne' I separated {'sodium `chromate in -the "presence `ofrai'carborraoeou'sreducing agent to form j'afproiiuctcomprising a sodium compound an'dlcalcium fciir'omite', 's eparati'ngthe sodium-'compound j from Lthe `'c'al'ciiun' chroniite, f' and Lreturning Lthe -vseparated "sodium VVcorrrpound -for "treatmentof u'Tatdlltifilal altel'edb'hrOmT/e. l

v""TQ'Ihe me'thdd'of "recoveririgbhromiumirom chromiteore'which *comprises 4"'forming a 4charge iofithe'ioreanid a basiccompound of the v'group "consisting 'of' lime -`anicl'fmagne'sia 'capableofsub- `f'st1tiiti`rig'for the ferrous `oxide 'of 'the ferrous )from *chemical *combination `with j the chromic "oxide ofthelferrous cinomiteandiformj an` altered chromite, heating vthe Jcharge 'to 'a "temperature 'contained theeinf'tofsodium chromate, converting the sodium vfchromate fto -sodium dichromata reducing the 4sodium dichromate with a `fcar -bonaceous vreducing agent, in the presence of calcium oxide, ito `Vform 'a `Aproduct Yco'nfiprisin'g 'sodium i arbonate and `a AWater1-insoluble -comlpound of chromium, Vseparating `the KAs'o'dil'im 'car- "bonate from the waterlinsolulole Achromium compound and returning the sodium carbonate ifor "further treatment `ofaltered chromite.

8. The 'method -o'f 'recovering chromium ffrorn 4chromiterore which -comprises formingia 'charge of 4the ore 4vand -a basic 'compound of "the group 'consisting of :lime 'and magnesia .capable of substituting Jfor fthe "ferrous joxider of the ferrous "chromite 'ofthe ore "to displace l"the -ferrous oxide from chenfiical combination nwith 'the chromic 'oxide'ofltheiferrous' chromite and form an altered chromite, 'heatingthe chargeto "a 'temperature above 120W C., 'and at least sufciently'high l'to `cause sintering tof constituents "of th'e 'charge i and 'to produce "a product containing an altered 'chromite formedfbysubstitution oi'thebasic compound for ferrous oxide ofthe chromitejoj fthe original .chromite `ore, decomposing vthe ,altered chromite 'digestion 'with ran aqueous solution of an alkali-'metal 'base,separ`atl lgthe aqueous Vsolution from the residual solid nmatten,subjecting the 'residualv solid'matter, `in 'the ,presence of `an alka'lie'metal compound, 4,to 'intimate Contact with an 'oxidizing Vg'as ,at Aa temperature, sufficiently Y 'highto convert chromium,containedtherin toan 'above"1"2'00C., cand atleast sufficiently/thigh to.,

cause sintering'fcoiistituents "of the"'cliarge"and fito Apifocluce a product A*containing an -altered chromiteformedibysujbstitution of the'basiccom- Y iritimateicontat Wuhanoxiuizingcgasat attemalkali-,mtal compound of chromium containing chromium in ithehexavaient stata leahinglthe nproduct of' 'the oxidizing treatment Withanaoue o'usfs'olution .of Acaustic .soda to .iorin .a ysolution fof sodium fchrmate, .separating `.the` solutioneof sodium chromate 'from residual :solid vmatter,

heating the solution .of `sodiumchromategtoecryy tallize -sodium .chromate'.therfrom, and reducing the .crystallized e sodium .chromfate @to produce :a sodiumbompound- :and a watereinsoluble .comgipo'u'nd of chromium. y

9. TheY ,method of recovering chromium from chromite ,ore which l comprises .iorming a ;charge of`lthe ore and .a lbasic compound. of -fthe sgroup .consisting .of lime andmagnesiafcapable ofssub- `stituting lfor the ferrous Voxide '.ofthe ferrous ',chromitefof'the .ore todisplace the :ferrousA oxide from .chemical .combination :with 'the chromic oxide of the ferrous chromite and formanaltered chromite, :heating =the charge tofa temperature Aabove vr1200o MAC., aand '-at leastvsufciently I:high to cause fsintering; of-.'constituents;.of the charge and .',tmproduc'e agproduct" containingamalterediohromite forrned sby :substitution of ''the A:basic com-- egpoundlorierrous foxide ,of ithelfchromite Jof the nriginalzzchromite zore, fdecomposing-r -Lthe faltered .-cmzomite loy-` digestion '.Withlan aqueous solution lfof ian alkalismetal ibase, separating the.i aqueous ian joxidizing u"gas at `ua tem'perature su'ciently Phigh" to f convert:chromium` contained therein *to #an alkali-metal compound 'of chromium vcontain- Y ling" chromium lin f' the ihexavalent state, leaching 'ithe'produce oi the5 oxidizing treatment with an Y aqueous -so'1ution"-o`f caustic'sodato formV ay solution'fvsodium-ichromate,separatingithe-solution f ofc'sofdiuml chromate'ifrom lresidualiso/lid' matter,

'ta'ilizeisodium,chromate therefrom; andreducing "the "crystallized =sddium vchromatewith a v"carbonaceous reducing agent to'producea sodium compound and chromic oxide.

10. The method of recovering chromium from chromite ore which comprises forming a charge of the ore and a basiccompound of the group consisting of lime and magnesia capable of substituting for the ferrous oxide of the ferrous chromite of the ore to displace the ferrous oxide from chemical combination with the chromic oxide of the ferrous chromite and form an altered chromite, heating the charge toa temperature above 1200 C., and at least suiiiciently high to cause sintering of constituents of the charge and to produce a product containing an altered chromite formed by substitution of the basic compound for ferrous oxide of the chromite of the original chromite ore, decomposing the altered chromite by digestion with an aqueous solution of an alkali-metal base, separating the aqueous solution from the residual solid matter, subjecting the residual solid matter, in the presence of an alkali-metal compound, to intimate contact with an oxidizing gas at a temperature sufliciently high to convert chromium contained therein to an alkali-metal compound of chromium containing chromium in the hexavalent state, leaching the product of the oxidizing treatment with an aqueous solution of caustic soda to form a solution o f sodium chromate, separating the solution of sodium chromate from residual solid matter, heating the solution of sodium chromate to crystallize sodium chromate therefrom, and reducing the crystallized sodium chromate in the presence of calcium oxide to form a product comprising sodium carbonate and calcium chromite.

11. The method of recovering chromium from chromite ore which comprises forming a charge of the ore and a basic compound of the group consisting of lime and magnesia capable of substituting for the ferrous oxide of the ferrous chromite of the ore to displace the ferrous oxide from chemical combination with the chromic oxide of the ferrous chromite and form an altered chromite, heating the charge to a temperature above 1200 C., and at least sufficiently high to cause sintering of constituents of the charge and to produce a product containing an altered chromite formed by substitution of the basic compound for ferrous oxide of the chromite of the original chromite ore, decomposing the altered chromite by digestion with an aqueous solutiony of sodium carbonate, separating the aqueous solution from the residual solid matter, subjecting the residual solid matter, in the presence of an alkali-metal compound, to intimate contact with an oxidizing gas at a temperature sufficiently high to convert chromium contained ytherein to an alkali-metal compound of chromium containing chromium in the hexavalent state, leaching the product of the oxidizing treatment with an aqueous solution of caustic ,soda to form a solution of sodium chromate, separating the solution of sodium chromate from residual solid matter, heating the solution of sodium chromate to crystallize sodium'chromate therefrom, reducing the crystallized sodium chromate with carbon in the presence of calcium oxide to form a product comprising sodium carbonate and calcium chromite, separating the sodium carbonate from the calcium chromite, and returning the sodium carbonate for further treatment of altered chromite.

12. The method of recovering chromium from chromite ore which comprises forming a charge of the ore and a basic compound of the group consisting of lime and magnesia capable of substimite, heatingv the chargeto atemperature above 1200" C., rand'at leastsufiiciently high to cause sintering of constituents of the charge and to produce a product containing an altered chromite formed by substitution of the basic compound for ferrous oxide of the chromite of the original chromite ore, decomposing the altered chromite by digestion with an aqueous solution of a compound from the class consisting of sodium hydroxide and sodium carbonate, separating the aqueous solution from thev residual solid matter, subjecting theresidual solid matter, in the presence of an alkali-metal compound, to intimate contact with an Aoxidizing gas at a temperature sufliciently high'to convert chromium contained therein to sodium chromate, bringing the product of the oxidizing treatmentfinto intimate contact with the -aqueous solution employed inthe digestion treatment of the altered chromite to vseparate sodium chromate from 'other materials associated therewith, concentrating such other materials to separate and recover undecomposed chromite, reducing the sodium chromate in the presence of calcium oxide to produce a sodium compound and chromic oxide in chemical combination with calcium oxide, and returning the sodium compound and the undecomposed chromite for mixing with further amount-s of additional altered chromite.'

13. The method of recovering chromium from chromite ore which comprises heating a charge of the ore and lime to a temperature above 1200 C., and at least sufficiently high to cause sintering of constituents of the charge and to kproduce a product containing an altered chromite formed by substitution of the lime for ferrous oxide of the chromite of the original ore, decomposing the altered chromite by digestion with an aqueous solution of a compound from `the class consisting of sodium hydroxide and sodium carbonate, separating the aqueous solution from the residual solid matter, subjecting the residual solid matter, in

Ythe presence of a sodium compound, to intimate f contact with an oxidizing gas at a temperature sufciently high to convert sodium contained therein to sodium chromate, separating sodium chromate from other materials associated therewith kin the product of the oxidizing treatment, concentrating such other materials to separate and recover undecomposed chromite, reducing the sodium chromate in the presence of calcium oxide to produce a sodium compound and chromic oxide in chemical combination with calcium oxide, separating the sodium compound, .and returning the separated sodium compound for further treatment of additional altered chromite.

14. The method of recovering chromium from chromite ore which comprises forming a charge of the ore and a basic compound of the group consisting of lime and magnesia capable of substituting for the ferrous oxide of the ferrous chromite of the ore to displace the ferrous oxide from chemical combination with the chromic oxide of the ferrous chromite and form an altered chromite, heating the charge to a temperature above 1200 C., and at least sufficiently highto cause sintering of constituents of the charge and to produce a, product containing an altered chromite formed by substitution of the basic compound for ferrous oxide of the chromite of the original chromite ore, decomposing the altered chromite by digestion with anV aqueous solution 

